1. Field of the Invention
The present invention relates to graphics system, and more specifically to a method and apparatus for recovering a clock signal associated with an analog display data received in a digital display unit (e.g., flat-panel monitor) of a graphics system.
2. Related Art
Digital display units are often used to display images. A flat-panel monitor generally used in lap-top computers is an example of such a digital display unit. A flat-panel monitor typically receives a source image from a graphics controller circuit and displays the source image. Flat-monitors which are being increasingly deployed with desk-top computers is another example of such a digital display unit. The source image is usually received in the form of analog data such as RGB signals well-known in the art.
Digital display devices often need to convert the received analog data into a sequence of pixel data. The need for such a conversion can be appreciated by understanding the general layout of a typical digital display device, which is explained below.
Digital display devices generally include a display screen including a number of horizontal lines. FIG. 1A is a block diagram illustrating an example display screen 100. Each horizontal line (shown as 101 through 106), in turn, is divided into several discrete points, commonly referred to as pixels. Pixels in the same relative position within a horizontal line may be viewed as forming a vertical lien (shown as dotted line 108).
The number of horizontal and vertical lines defines the resolution of the corresponding digital display device. Resolutions of typical screens available in the market place include 640.times.480, 1024.times.768 etc. At least for the desk-top and lap-top applications, there is a demand for increasingly bigger size display screens. Accordingly, the number of horizontal display lines and the number of pixels within each horizontal line has also been generally increasing.
Thus, to display a source image, the source image is divided into a number of points and each point is displayed on a pixel. Each point may be represented as a pixel data element. Display signals for each pixel in display 100 may be generated using the corresponding display data element. However, as noted earlier, the source image may be received in the form of an analog signal. Thus, the analog data needs to be converted into pixel data for display on a digital display screen.
It is helpful to understand the typical format of the analog data to appreciate the usual conversion process. Generally, each source image is transmitted as a sequence of frames, with each frame including a number of horizontal scan lines. Image is generated on display screen 100 by displaying these successive frames.
Usually, a time reference signal is provided in parallel to divide the analog signal into horizontal scan lines and frames. In the VGA/SVGA environments known in the art, the reference signals include VSYNC and HSYNC. The VSYNC signal indicates the beginning of a frame and the HSYNC signal indicates the beginning of a next source scan line. The relationship between HSYNC and the analog signal data is illustrated further with reference to FIG. 1B.
Signal 150 of FIG. 1B represents an analog display data signal in time domain. Analog signal 150 represents a display image to be generated on display screen 100. The display signal portions 103B, 104B, 105B etc. represent display data on corresponding horizontal lines 103A, 104B, and 105B respectively. The portions shown as straight lines correspond to a `retrace` period, which signifies the transition to a next horizontal line.
Such transitions are typically indicated by another signal (e.g., HSYNC signal in computer displays). Pulses 103B, 104B, and 105B represent such transitions. Thus, after a transition, the display portion of the signal may be sampled a number of times. The exact number may be proportional to the number of pixels on each horizontal line on display screen 100. Each display portion is generally sampled the same number of times to generate samples for each pixel.
Thus, to convert the source image received in analog signal form to pixel data suitable for display on a digital display device, each horizontal scan line is converted to a number of pixel data. For such a conversion, each horizontal scan line of analog data is sampled a predetermined number of times. The sampled value is represented as a number, which constitutes a pixel data element.
Each horizontal scan line is typically sampled using a sampling clock signal. That is, the horizontal scan line is usually sampled during each cycle of the sampling clock. Accordingly, the sampling clock is designed to have a frequency such that the display portion of each horizontal scan line is sampled a desired number of times. The desired number can correspond to the number of pixels on each horizontal display line of the display screen. However, the desired number can be different that the number of pixels on each horizontal display line.
Using the sampling scheme described above, each horizontal scan line of a source frame is represented as a number of pixel data. It will be readily appreciated that the relative position of source image points needs to be properly maintained when displaying the source image. Otherwise, some of the lines will appear skewed in relation to the other on the display screen.
To maintain a proper relative position of the source image pixels, the sampling clock may need to be synchronized with the reference signal. That is, assuming for purposes of explanation that HSYNC signal is used as a time reference, the beginning of sampling of analog data for a horizontal display line may need to be synchronized with HSYNC signal pulse. Once such a synchronization is achieved, the following pixels in the same horizontal lines may also be properly aligned with corresponding pixels in other lines.
Phase-locked loop (PLL) circuits implemented using analog components have conventionally been used to achieve such a synchronization. FIG. 2 is a block diagram of an example PLL circuit 200 which is implemented for such a synchronization. In addition, PLL circuit 200 generates the sampling clock signal also. PLL circuit 200 includes phase detector 210, filter 220, amplifier 230, voltage controlled oscillator (VCO) 240, and frequency divider 250. Phase detector 210 compares a time reference (e.g., VSYNC) received on line 102 and sampling clock (more accurately, a signal having a predetermined fraction of the sampling signal) received on line 251. The two signals are referred to as f1 and f2 for brevity.
Phase detector 210 provides on line 212 a signal having a difference of the frequencies of f1 and f2. The signal on line 212 may also include several harmonics of the difference frequency. Filter 220 is generally designed as a low pass filter to eliminate undesirable components. When the frequencies f1 and f2 are close, but not equal, line 223 will carry a signal with the difference frequency. VCO 240 is designed to generate a signal with a predetermined frequency. However, the frequency is altered depending on the voltage level received on line 234.
Amplifier 230 amplifies the signal on line 223 to provide a desired level of voltage on line 234 to modify the frequency of VCO 240. The voltage level is generated so as to achieve a synchronization of the frequencies f1 and f2. Frequency divider 250 divides the frequency of clock signal received on line 245 by a factor of n. By choosing an appropriate value of n, analog signal data for each horizontal source scan line can be sampled a desired number of times. The signal on line 245 can be used for such a sampling.
However, it is well known in the art, the reference frequency (HSYNC) can vary by a slight value from an average frequency during normal operating conditions. In addition, the reference frequency can drift over a prolonged period of time due to, for example, temperature changes in the circuits generating the analog source image data. Further, jitter may be present in both the reference signal and the clock signal generated by the analog PLL.
In general, it is desirable that the PLL of FIG. 2 track the long term drifts while eliminating the jitters. This may be achieved by having a PLL circuit with low bandwidth (e.g., 100 to 1000 Hz). However, such a low bandwidth generally requires a capacitor having a large size, which may be hard to integrate into a relatively small-sized integrated circuits.
Some prior approaches have placed the capacitor external to the integrated circuit, with the capacitor being coupled to the integrated circuit by pads. One problem with this approach is that noise is introduced into the analog PLL loop due to the external couplings. Analog PLLs are generally sensitive to such noises, leading to instability in the PLL loop. Without a low bandwidth in the loop, PLL 200 may be unable to track deviations in the reference signal closely, which may be unacceptable in some situations as explained below.
Deviations of about 5 to 20 nano-seconds in time reference period can be common in a typical graphics environment. These deviations are usually more problematic for larger size display screens. To illustrate this point with an example, a 640.times.480 size display screen has a pixel processing period (i.e., average time to display each pixel) of 40 nano-seconds, while a large 1280.times.1080 size monitor can have a pixel processing period of about 8-9 nano-seconds. A deviation of 20 nano-seconds may not have a perceptible impact on the display of a 640.times.480 screen due to the relatively larger pixel processing period, whereas the same amount of deviation can cause the display on the large monitor to be skewed by two pixels.
Such a skew between lines is generally perceptible for the human eye and the resulting display quality may be unacceptable. The display quality is further exacerbated if the number of such skews is larger. As is well known in the art, the display quality problems can be ameliorated by a circuit which can track the time reference signal more closely. Therefore, what is needed is a circuit which tracks the time reference signal closely.